Bacteria like Salmonella survive within macrophages in a membrane-enclosed vacuole, or compartment. Being within the macrophage allows them to avoid detection by other immune cells, such as bacteria-killing neutrophils. But macrophages only live about 30 days, so intracellular bacteria like Salmonella must periodically find a new host cell. And they need to do so without being detected and destroyed by the immune system.
The researchers found that the Salmonella bacteria that have infected a macrophage use particular weapons — virulence factors — to create holes in the membranes of both the vacuole and the macrophage to trigger macrophage death, thereby opening channels to the outside. These holes allow the bacteria to activate the complement system and generate a “find-me” signal to attract neutrophils to find bacteria entrapped in dying macrophages.
When a neutrophil arrives to clean up the dead macrophage with the Salmonella inside, it engulfs both the dead macrophage and the Salmonella in a process called efferocytosis. The dead macrophage then shields the Salmonella from the antimicrobial properties of the neutrophil, allowing the bacteria to survive.
Thus, Salmonella hijacks key aspects of the host immune system, namely the complement system and neutrophils, in order to persist within the infected host.
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